Many private schools fail to check teachers' criminal records, while others have serious health and safety issues and poor curricula, chief inspector David Bell's annual report reveals.
Four out of 10 of the private schools inspected by Ofsted failed to meet statutory requirements, with a fifth employing staff before checking them properly.
"There was an increase in the number of schools employing staff without the required checks on their suitability having been carried out," said the report.
Private school accommodation had generally improved but the report raised "serious concern" about some with worries about fire safety and risks from building work.
"A number of provisionally registered schools await approval of their fire safety arrangements," said the report, published last week.
"Elsewhere, action has been required by the Health and Safety Executive to protect children from building works and other risks."
Past Ofsted reports on private schools have highlighted health and safety issues such as broken glass in play areas, exposed wiring, broken toilets and food waste not properly thrown away.
Mr Bell was also concerned about the curriculum in some private religious schools and theatre schools.
He said in a minority of religious schools insufficient time was allocated to the secular curriculum while in some specialist arts schools, especially those concentrating on drama, the curriculum remained narrow.
The findings do not apply to members of the Independent Schools Council, such as Eton and Harrow, which has its own inspection body. Six out of 10 private schools are ISC members and they teach 80 per cent of children in the private sector.
ISC spokesman Dick Davison said: "Anything affecting the reputation of private schools is of concern, but 80 per cent of children in the independent sector are educated in ISC schools, where standards are very closely regulated."